Online Watching Prons Why Does The Audio Not Match Up On A Video I Am Watching Online?

Why does the audio not match up on a video I am watching online? - online watching prons

This does not happen very often, but everynow and then I'll watch a video online, and suddenly realize that the sound is not synchronized with the video (It can sometimes be a delay of 10 seconds). Once that happens, it does not seem possible to rectify the situation. First, I wonder why this is happening, and secondly, there is a way to prevent this. Any help would be awesome!